In castable flow molding processes employing a plurality of castable polyurethane components, the homogeneity and the quality of the molded material is mainly determined by the mixing operation which immediately precedes the molding.
For example, after an amount of time in which the reactants come into contact, a polymerization reaction process begins producing the moldable material. Many times, striae form within the moldable material that is visible. The striae are a result of poor mixing which inhibits the quality of the material. Therefore, it is desirable to produce a mixture which is as homogeneous as possible, in the shortest possible time, in order to bring about a uniform reaction to avoid the formation of striae. However, there is an additional difficulty presented in mixing reactive components in the case of polyurethane, in that the two components, i.e., polyol and the isocyanate, have substantially different viscosities.
The use of known mixing processes does not lead to the desired result for producing a high quality polyurethane material. For example, with some processes that employ static mixers that make use of various known mixers for mixing liquids in the laminar flow regime, it is found that a relatively long mixing length is needed to produce sufficient mixing. Often, the mixture requires a relatively long time to pass through this long mixing length, meanwhile, the polymerization process has already begun. Due to the quick setting characteristics of polyurethane, the material will gel or “set up” within the mixer instead of being discharged into the usual succession of molds. The molds are generally moved past the discharge of the mixer in time relation to the discharge. If, for any reason, a slight delay or decrease in the flow rate of the mixture through the mixer occurs, the mixture gels in portions of the mixer and restricts flow, thus further slowing the discharge and resulting in the entire mixer being clogged with hard setting components. An improvement in slowing down the gel time is necessary to allow the mixture to progress through the system.
Generally, static mixers are in the form of a tubular chamber, with a rigid static mixing device disposed therein. Because of the very nature of the static mixer, the mixer cannot be cleaned readily once any appreciable quantity of material has gelled in the various mixing elements which form the static mixing device. Attempts have been made to clean the static mixer, but due to the cementing and interlocking effects of the material this approach has proven impractical. Therefore, available static mixers perform poorly in practice because the mixer may only be used, in some instances, for 15 to 30 minutes before “plugging-up”.
If in place of the static mixer, a dynamic mixer may be employed with the aim of reducing the mixing time. While the results generally improve the quality of mixing, the temperature of the reaction mixture may be increased by frictional and shear heating, and local fractions of the mixture which can be generated in an advanced state of polymerization must be eliminated. Consequently, when dynamic mixers are used, significant improvements must be made towards controlling the exothermic temperatures. Additionally, caution must be taken to insure that the dynamic mixer does not introduce pockets of gas in the form of air bubbles into the moldable material, which may lead to poor quality. Moreover, dynamic mixers may require frequent flushing with solvents resulting in a sludge material which has to be disposed of.
The present invention is directed to overcoming one or more of the problems as set forth above.